Israeli immigration officials at Tel Aviv airport secretly copied the British passports which were then used by the hit squad which assassinated a leading Hamas official, ministers have been told.

The six British citizens whose identities were stolen and used by the killers all had their passports taken away from them briefly during routine checks at the airport, it has been claimed.

The revelation by diplomatic sources that the Foreign Office has been told that the passports were copied by Israeli officials is the first time Israel’s involvement has been directly alleged.

It will put further pressure on the Israeli government which has been at the centre of a growing diplomatic storm over its possible involvement in the murder of Mahmoud al-Mabhouh in Dubai last month.

Diplomatic sources say ministers have been briefed that the passport fraud was committed by immigration officials who stopped the British nationals, who all now live in Israel, as they went through the airport during recent trips.

The passport numbers were taken, most likely by photocopy, and then used to create new documents which were used by the hit squad.

The identities of French, German and Irish citizens were also used.

The suspects used fake passports bearing their own pictures, but the names and numbers of the innocent Europeans.

All six British passports were not biometric, which means they did not have a computer chip embedded in them and so the fraud would have been relatively straightforward, experts believe.

The revelation will put further pressure on Israel to come clean about what it knows following widespread speculation about the involvement of Mossad, the Israeli intelligence agency, in the killing, although other organisations have not been ruled out.

Mabhouh was one of the founders of Hamas’s military wing and has been wanted by Israel for his role in the 1989 kidnapping and killing of two Israeli soldiers on leave.

He was murdered in a Dubai hotel room in January after an elaborate plot involving at least 11 assassins, including at least one woman, posing as tourists, with some wearing wigs and false beards.

Six holders of British passports, three with Irish documents, one with a German passport and another with a French passport made up the hit team.

David Miliband, the Foreign Secretary, held talks with the Israeli ambassador to London, Ron Prosor, last week as the row threatened to escalate into a major diplomatic crisis.

He denied that the Government was merely “going through the motions” and described the use of fake British passports in the assassination as an “outrage”.

But the Foreign Office said it was too early to speculate on who could have carried out the identity theft.

Michael Levi, a professor of criminology at Cardiff University and an expert on identity theft, said the passports would not have been difficult to tamper with.

He said: “The sort of organisation that can pull off a hit like that will be able to make those sort of changes to a passport.”

The Conservative leader, David Cameron, said Israel must provide assurances that it would never sanction the use of UK papers in operations by its secret service.

He also called for answers from the Government about when it knew that falsified documents were used in the murder on January 20. Reports last week alleged that ministers may have had a prior tip-off.